how to automatically fulfill the shipping address in the checkout page?

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I am developing an embedded app which automatically fulfills shipping address for customers in the checkout page. I conformed my app can get the default address of merchant by graph query, but not sure how to catch an event when customers click 'checkout' and fulfill shipping address automatically. 


I found an app - Zapiet is doing the same thing - when the customer clicks a local pickup it automatically fills up the store location in checkout page, so there must be a way to do that. Please shed on the light for me.